1. Field of the Invention
The present invention relates to an objective optical system which has a zooming function.
2. Description of Related Art
In general, medical endoscopes are used for their purposes. A first lens in an objective optical system is disposed in an end section of a tip and other lenses and an image-capturing element such as a CCD are disposed on an optical axis of the first lens in a linear manner in a direct view type endoscope. On the other hand, the first lens is disposed on a side section of a tip of the endoscope and an optical axis of this first lens is bent by a prism, etc., and other lenses and a CCD are disposed on a bent optical axis in a side-view type endoscope. Thus, these endoscopes are used separately according to purposes such as a point to where the endoscope is supposed to be inserted into a human body and a purpose of the medical examination.
It is required that an outer diameter of the endoscope should be as small as possible so as to alleviate a pain of a patient. In order to realize such a requirement, it is certainly necessary to reduce a size of an objective optical system.
In general, an optical axis direction converting element such as a prism, etc, is necessary for this side-view type endoscope. It is preferable that the optical axis direction converting element should be disposed near a first lens and diaphragm of which height of a light should be lower in order to prevent that a diameter of the endoscope may be reduced.
For such an objective optical system in this type of side-view type endoscope, an optical system is known which is described in Japanese Unexamined Patent Application, First Publication No. Sho 51-62053 in pages 2 to 6, FIGS. 1 to 9). In these embodiments each member are disposed as follows. A first lens group which has a negative refractive power is disposed nearest to the object. A prism is disposed second nearest to the object. A diaphragm is disposed third nearest to the object. A second lens group which has a positive refractive index is disposed the farthest to the object.
In addition, among the endoscopes, what is called a magnifying endoscope of which is purpose is to perform a diagnostic operation for a degree of a slight lesion and a range which is supposed to be executed by magnifying a lesion part and observing a fine structure is drawing attention. For such a side-view type objective optical system in this magnifying endoscope, an optical system is known which is described in Japanese Unexamined Patent Application, First Publication No. Hei 1-279219 is known (pages 4 to 8, FIGS. 1 to 4). In these embodiments, the first lens group which has negative refractive power is disposed nearest to the object. The diaphragm is disposed second nearest to the object. The moving lens group which has a positive refractive power is disposed the farthest from the object such that a focal point has the greatest proximity simultaneously when the focal length is longer; thus, an magnified view is obtained in a telephoto end because the moving lens group moves toward the diaphragm on the optical axis. That is, a lens system which performs a magnification and a focusing operation simultaneously in a single operation is realized.
In the above conventional objective optical system, a diameter of the lens in the objective optical system for the side-view type endoscope which is described in Japanese Unexamined Patent Application, First Publication No. Sho 51-62053 becomes great because a height of a ray in a moving lens group becomes high in a wide-angle end in which the moving lens group is disposed from the diaphragm in the farthest distance. Nonetheless, in any cases, the moving lens group approaches the optical axis direction converting element and a diaphragm at the telephoto end.
Also, a prism is disposed between the first lens group and the moving lens group in the objective optical system of the side-view type endoscope which is described in the Japanese Unexamined Patent Application, First Publication No. Hei 1-279219. The image-capturing element such as a CCD in a side-view type electronic endoscope is necessary to be contained sufficiently within an outer wall of the endoscope not at an edge of the outer wall because of securing a strength therein. Therefore, an optical axis of the first lens which is bent by the optical axis direction converting element is necessary to be disposed sufficiently inside of the endoscope. Also, an inner diameter of a lens barrel in which a lens behind the diaphragm is supposed to be contained is necessary to be small by a certain distance because there is a frame for receiving the first lens from a structural point of view of the mirror frame.